if I could edit I would take out Pam in this last sentence.
now, I haven't read all the comments, so I'm not sure if this has been touched on. I think you're right in saying that the Office has become more and more depressing and Jim's character has kind of slipped into a middle management, mediocre lifestyle rut with no real friends...However, are we really surprised by this? There were always references to how not capable Jim was at doing his job, or doing Michael's job. The times Michael left him in charge and Jim found out that maybe Michael had to do more than he was given credit for and this was harder than he thought it was. The subtle moments between Michael and Jim where Michael sort of implies, that someday, when Jim gets up there, he'll get the hang of it. While it is depressing that Jim sort of represents this "worst purgatory of 30-something life", I never felt like Jim was ever going to amount to more than that, and, because of his lack of enthusiasm for anything other than Pam and pranks, he probably would never really try. Maybe I'm just a cynic with a cold heart of stone, but was Jim or Pam ever really meant for anything more?